This application discloses an invention which is related, generally and in various embodiments, to a system and method for controlling an electric motor.
FIG. 1 illustrates a high level representation of an electric propulsion drive system 10. The system 10 includes a variable frequency motor drive 12, an electric motor 14, and a vector control system 16. In general, the vector control system 16 may be implemented at least in part by a computing device (e.g., a vector or field oriented controller). As shown in FIG. 1, the variable frequency motor drive 12 is electrically connected to a constant frequency AC source 18, and the electric motor 14 is electrically connected to the variable frequency motor drive 12. The electric motor 12 is mechanically connected to a shaft 20, and the shaft 20 is also mechanically connected to a propeller 22. For purposes of simplicity, the system 10 will be described in the context of a three-phase system.
In operation, the constant frequency AC source 18 applies a three-phase fixed frequency AC voltage to the variable frequency motor drive 12. The variable frequency motor drive 12 draws a three-phase fixed frequency alternating current then applies a three-phase variable frequency AC voltage to the electric motor 14. The electric motor 14 draws a three-phase variable frequency alternating current which causes a portion of the electric motor 12 to rotate, thereby causing the shaft 20 and the propeller 22 to rotate. The rotation of the propeller 22 operates to propel a vessel in a given direction.
To provide control of the torque applied to the shaft 20 and the speed of the shaft 20, at least two of the individual phase currents drawn by the electric motor 14 are sensed or measured then provided as inputs to the vector control system 16. The rotational speed and angle of the shaft 20 are also sensed or measured, then provided as additional inputs to the vector control system 16. Based on the sensed or measured phase currents drawn by the electric motor 14 and the sensed or measured rotational speed and angle of the shaft 20, the vector control system 16 generates three AC phase voltage reference signals that are provided as inputs to the variable frequency motor drive 12. Based on the generated AC phase voltage reference signals provided as inputs to the variable frequency motor drive 12 by the vector control system 16, the variable frequency motor drive 12 adjusts the three-phase variable frequency AC voltage applied to the electric motor 14 so that the desired torque and shaft speed are realized.
FIG. 2 illustrates a simplified 2-pole representation of a permanent magnet synchronous motor. In a typical Field Oriented Vector (FOV) control system for a permanent magnet motor, the stator current and stator voltages are transformed to a rotating reference frame (e.g., DQ reference frame) which is synchronized with the permanent magnet flux of the rotor as shown in FIG. 2. The direct axis (d-axis) is oriented with the permanent magnet flux and the quadrature axis (q-axis) is 90 degrees out of phase from the direct axis. In this rotating reference frame, it has been well established that the motor torque is given by the following equation:
                              T          =                                    3              2                        ⁢                          poles              2                        ⁢                          (                                                                    λ                                          d                      ⁢                                                                                          ⁢                      s                                                        ⁢                                      i                                          q                      ⁢                                                                                          ⁢                      s                                                                      -                                                      λ                                          q                      ⁢                                                                                          ⁢                      s                                                        ⁢                                      i                                          d                      ⁢                                                                                          ⁢                      s                                                                                  )                                      ,                            (        1        )            or alternately, by the following equation:
                    T        =                              3            2                    ⁢                                    poles              2                        ⁡                          [                                                                    λ                    pm                                    ⁢                                      i                    qs                                                  +                                                      (                                                                  L                        md                                            -                                              L                        mq                                                              )                                    ⁢                                      i                    qs                                    ⁢                                      i                    ds                                                              ]                                                          (        2        )            where T is the motor electromagnetic torque, poles are the number of motor poles, λds is the stator d-axis flux linkages, iqs is the stator q-axis current, λqs is the stator q-axis flux linkages, ids is the stator d-axis current, λpm is the rotor permanent magnet flux linkages, Lmd is the d-axis magnetizing inductance, and Lmq is the q-axis magnetizing inductance. The stator d-axis and q-axis flux linkages are given by the following equations:λds=pm+(Lls+Lmd)ids  (3)andλqs=(Lls+Lmq)iqs  (4)where λpm is the permanent magnet flux, Lls is the leakage inductance of the electric motor, and the other quantities are as indicated above.
It should be noted that FIG. 2 illustrates the specific case of positive d-axis stator current (ids>0), where the d-axis flux linkage from the permanent magnet or field coil is increased by the stator d-axis current. For purposes of simplicity, this is the convention utilized herein for the motor fluxes and currents. However, there is nothing to limit the stator d-axis current to positive values or to prevent the stator d-axis current from being negative and opposing (or bucking) the rotor flux. Negative d-axis current is a common mode of operation at high speed and torque loading since the stator winding flux linkage and motor voltage often become excessive without any additional flux reduction or “field weakening”. The flux reduction is generally achieved by driving the d-axis current negative, thereby reducing the overall d-axis flux linkage. The reduced d-axis flux linkage, in turn, decreases the total stator winding flux linkage and consequently the motor terminal voltage.
The typical goal of the vector control system 16 is to align the rotating reference frame associated with the transformation to direct and quadrature axes so that the direct axis coincides with the positive rotor flux and the quadrature axis is rotated from the direct axis by 90 degrees in the counter clockwise direction. Since all the rotor flux is directed down the d-axis, the quadrature axis flux generated by the rotor is zero. Torque control can then be obtained by keeping the direct axis flux (λds) nearly constant and operating only on the quadrature axis current (iqs). The direct axis flux is provided primarily by the permanent magnets on the rotor, but can be adjusted with ids to maintain motor terminal voltage within allowable limits for a variety of load conditions.
FIG. 3 illustrates a more detailed representation of the electric propulsion drive system 10 of FIG. 1. As shown in FIG. 3, the vector control system 16 receives the following signals as inputs: a shaft angle signal, a shaft speed signal, and at least two motor phase currents. The shaft angle is used in the transformation from phase variables to the rotating DQ reference frame. If the q-axis flux is assumed to be small, the torque is primarily controlled by adjusting the stator q-axis current, iqs.
The vector control system 16 in FIG. 3 assumes that the motor flux is aligned with the permanent magnet flux along the d-axis as shown in FIG. 2. In this case, the motor flux depends only on the permanent magnet flux λpm and ids. However, this is only the case when the torque is zero. At torque values other than zero, the motor flux depends on both the d-axis and q-axis stator currents, and is shifted from the d-axis as shown in FIG. 4. For cases when the torque values are other than zero, the magnitude of the motor magnetizing flux is given by the following equation:
                                                    λ            m                                    =                                            λ              md              2                        +                          λ              mq              2                                                          (        5        )            where λmd=λpm+Lmdids and λmq=Lmqiqs.
Thus, for the vector control system 16 of FIG. 3, the flux linkage is dependent on both the d-axis and q-axis current components. Further, the torque is also dependent on both the d-axis and q-axis currents if a reluctance torque component (i.e., Lmd≠Lmq) is present.